Recently, as a rewritable nonvolatile memory is noted a magnetic random access memory (herein after called “MRAM”) including magnetoresistive effect elements arranged in a matrix. MRAM uses combinations of the magnetization directions of 2 magnetic layers to store information and detects resistance changes (i.e., current changes or voltage changes) given when magnetization directions of the magnetic layers are parallel are antiparallel to read stored information.
Recently, memory cells which can store multivalued information are proposed (T. Ishigaki et al., “A Multi-Level-Cell Spin-Transfer Torque Memory with Series-Stacked Magnetotunnel Junctions,” 2010 Symposium on VLSI Technology Digest of Technical Papers, pp. 47-48).
Related references are as follows:    Japanese Laid-open Patent Publication No. 2002-100192;    T. Ishigaki et al., “A Multi-Level-Cell Spin-Transfer Torque Memory with Series-Stacked Magnetotunnel Junctions,” 2010 Symposium on VLSI Technology Digest of Technical Papers, pp. 47-48;    K. Tsuchida et al., “A 64 Mb MRAM with Clamped-Reference and Adequate-Reference Schemes,” 2010 IEEE International Solid-State Circuits Conference Digest of Technical Papers, pp. 258-259;    A. Bette et al., “A High-Speed 128 Kbit MRAM Core for Future Universal Memory Applications,” 2003 Symposium on VSLI Circuits Digest of Technical Papers, 4-89114-035-6/03;    Yoshihiro Ueda et al., “Design of Low Read Bias Voltage and High Speed Sense Amplifier for STT-MRAM,” The Institute of Electronics, Information and Communication Engineers, IEICE Technical Report, 2007, Vol. 107, No. 1, pp. 7-12; and    Young Min Lee et al., “Highly Scalable STT-MRAM with MTJs of TOP-pinned Structure in 1T/1MTJ Cell,” 2010 Symposium on VLSI Technology Digest of Technical Papers, pp. 49-50.